Pressure-sensitive adhesive tapes and sheets produced with pressure-sensitive adhesive compositions conventionally used can form a bond at room temperature when a force on the order of a finger pressure is applied to them. Therefore, they are used in a wide variety of fields such as bonding, wrapping, surface protection, and electrical insulation. Particularly in bonding applications, good adhesion is often required for reliability and safety.
In recent years, for example, cellular phones, digital cameras, PDAs, digital video cameras, and other OA devices, and electronic components (particularly mobile devices) have increased in functionality and decreased in size and thickness as they have become widespread and increased in production. For example, cellular phones as typical mobile devices tend to have thinner main components so that they can be easier to carry and have a wider display screen.
In general, a display part composed mainly of an LCD module and a backlight unit has various many sheet-shaped components (placed on one another) for performing functions such as light emission, reflection, light shielding, and light guiding. Double-sided pressure-sensitive adhesive tapes and the like are used in assembling (bonding) these components (see Patent Document 1).
Silicone-based pressure-sensitive adhesives, acrylic pressure-sensitive adhesives, and the like are typically used as raw materials for double-sided adhesive tapes. Unfortunately, silicone-based pressure-sensitive adhesives are problematic in that they are expensive and less economical.
Pressure-sensitive adhesive sheets are also used in assembling (bonding) of mobile device components and in boding of an optical member such as a lens (e.g., a glass or plastic lens), a prism, a reflector, a compensation plate, or a polarizing plate. Acrylic pressure-sensitive adhesives are mainly used for such pressure-sensitive adhesive sheets (see Patent Documents 1 and 2).
In recent years, as parts and components have decreased in size and thickness, pressure-sensitive adhesive tapes have become required to have high adhesion even to a small area. For design features and the like, more components are having a curved surface, which creates a demand for high adhesion reliability during bonding to such a curved surface.
However, for example, when the acrylic pressure-sensitive adhesives are used, a large amount of organic solvents, which have been used to form the adhesives, need to be removed by heating and drying, which reduces the workability and may have an adverse effect on the environment. In addition, when pressure-sensitive adhesive tapes are produced using an acrylic pressure-sensitive adhesive solution containing a large amount of an organic solvent, it is difficult to form pressure-sensitive adhesive layers by thick coating, which causes a problem in that their applications will be limited due to limitations on the thickness of pressure-sensitive adhesive layers.
Although inexpensive, acrylic pressure-sensitive adhesives may contribute to the problem of petroleum resources depletion because petroleum is often used as a raw material for them. In addition, carbon dioxide is emitted in the process of disposal of acrylic pressure-sensitive adhesives after use, which is not global-environmentally conscious. There has been a demand for measures against global warming.